1. Field of the Invention
The present invention relates to an object lens actuator which drives an object lens for focusing light on a recording surface of an optical disc in focusing and tracking directions so as to reproduce information recorded on the recording surface of the optical disc or record information thereon and to a disc drive using the same.
2. Background Art
A disc drive for recording information on a disc-like information recording medium or reproducing the recorded information is designed to increase the data transfer rate by rotating an optical disc at high speeds. The disc drive includes an object lens actuator which drives an object lens in focusing and tracking directions so as to accurately record or reproduce information by following the rotation of the optical disc.
A general object lens actuator comprises: a magnetic circuit having a yoke and a permanent magnet; a movable portion to which an object lens is attached; a fixing portion for holding the movable portion; and first and second support members which are connected to the fixing portion and resiliently support the movable portion. The object lens is disposed on a center line between the first support member and the second support member. Additionally, a focusing coil and a tracking coil are attached to a flat plate between the two opening portions disposed in the movable portion. When current is supplied to the focusing coil, the movable portion is driven in a focusing direction by an electromagnetic force generated by an effect of magnetic flux from a permanent magnet attached to the yoke. Likewise, when current is supplied to the tracking coil, the movable portion is driven in a tracking direction by an electromagnetic force generated by an effect of magnetic flux from a permanent magnet attached to the yoke (see JP Patent Publication (Kokai) No. 8-194962 (1996)(FIG. 1)).
Such an object lens actuator needs to enhance the control band for focusing and tracking with an increased number of rotations of the optical disc so as to position the object lens in focusing and tracking directions. To provide a stable control, the amplitude in a higher order resonance frequency of the object lens actuator needs to be small.
There is a torsional vibration of the lens holder as a higher order resonance of the object lens actuator. According to the conventional technique, when the object lens are placed symmetrically with respect to the center line between the first support member and the second support member which resiliently support the lens holder therebetween respectively, the object lens actuator is hard to be subject to torsional vibration. However, when the object lens are biased toward the inner periphery with respect to the center line, viewed from an optical axial direction of the object lens and between the support members which resiliently support the lens holder therebetween, so as to enable access to a data area provided at the inner periphery of an optical disc, the object lens actuator may have a problem in that when torsional vibration of the lens holder occurs, the position of the object lens serves as a vibration loop which may increase the amplitude thereof.